There are many possible ways to build the
hardware for a quantum computer, because there are many physical systems that can be used as qubits. But to give you a concrete example, I’m going to show you what it takes to build a quantum computer using the spin of single electrons inside a solid state semi-conductor chip. This is a pathway that is being pursued very actively partly because it may allow us to build quantum computers using the same fabrication methods used for classical computer
chips. But the way the quantum chip is operated will be very different. First of all, we need to confine individual electrons within a chip. This is actually not that hard. The latest
silicone chips in your computer and mobile phone have transistors that are only 14 nanometers wide. That’s about 40 silicone atoms across. And with some minor modifications, you can use the same technology to build electronic devices where all the electrons are squeezed
out except for one, which then becomes our qubit. The quantum information is encoded
in the spin of the electron. To be able to distinguish between the 0 and the 1 state of the spin, we need to make sure that the energy difference between them is larger than the thermal energy of the environment. The energy difference between the up and down spin states is proportional to the magnetic field applied to the spin. With a magnetic field of 1 tesla which is twenty thousand times stronger than the Earth’s magnetic field the energy difference is about one degree kelvin. So we need to produce very large magnetic fields and very low temperatures, close to absolute zero, for this to work. Then, we need to perform quantum logic operation with these electron spin quantum bits. For a single qubit operation, we use magnetic resonance, the same method used in MRI scanners. Because of the very high magnetic fields we need to use, the frequency of electromagnetic excitation is also very high. It’s tens of gigahertz which is the range typically used by a radar for example. For two qubit operations, we need to control the interaction between nearby electrons. There are many ways to do this, including a method which consists of making all bits of the electron overlap in a similar way as what happens in chemical bonds within molecules. So in a sense, we are creating artificial molecules within a chip, with the bonds controlled by the voltage of tiny electrodes fabricated on the chip. So overall, the hardware to build and operate a quantum computer consists of a special silicone chip with very small transistors that hold just one electron each, a machine that produces large magnetic fields and extremely low temperatures, high frequency electronics to implement magnetic resonance control of the state of the spins and tiny electrodes to control the interaction between neighbouring spins. This is not science-fiction. All these things exist, we know how to build them and how to develop them. We are now at the stage where we can scale up quantum computer chips and soon enough, reach the point where the exponential power of quantum computers can be used for novel applications.